1. Field of the Invention
The present invention relates generally to a running control of an automobile or motor vehicle and more particularly to an apparatus for controlling the running of a motor vehicle by controlling a throttle valve of an internal combustion engine with an electric signal generated in response to actuation of an accelerator pedal.
2. Description of the Related Art
The intake air flow fed to a gasoline engine of a motor vehicle is conventionally regulated by a throttle valve whose opening is controlled by means of an accelerator pedal mechanically interlocked with the throttle valve. In recent years, however, there has been developed and adopted in some practical applications a so-called drive-by-wire system in which the opening of the throttle valve is controlled by an electrical throttle actuator in response to an output signal of an accelerator pedal position sensor which signal represents depth or stroke of depression of an accelerator pedal of the motor vehicle, with a view to enhancing the comfortableness in driving the motor vehicle and realizing a driving at a cruising speed.
For a better understanding of the present invention, the background technique thereof will be described in some detail.
FIG. 6 shows generally and schematically a structure of a motor vehicle running control apparatus disclosed in Japanese Unexamined Patent Application Publications Nos. 1687/1968 and 76236/1982 (JP-A-43-1687 and JP-A-57-76236). Referring to the figure, an internal combustion engine (hereinafter also referred to as the engine) denoted by a reference numeral 10 is equipped with an intake pipe 9 in which a throttle valve 12 is installed for controlling or adjusting the amount of air supplied to the engine 10. The throttle valve 12 is provided with a return spring 11 for allowing the throttle valve 12 to assume a fully closed position, when it is released. An electromagnetic clutch 62 has an output shaft coupled to a rotatable shaft 12a of the throttle valve 12 and an input shaft coupled to a driving mechanism 61. The electromagnetic clutch 62 and the driving mechanism 61 cooperate to constitute a throttle actuator 6 adapted to be controlled by a control unit 4 which includes an actuator control circuit 41, a fault detection circuit 42 and a clutch control circuit 43. An accelerator pedal depression depth or stroke sensor 1 is provided, being interlocked to an accelerator pedal 14 for generating a voltage signal proportional to the depth of depression (stroke of actuation) of the accelerator pedal 14. The voltage signal outputted from the sensor 1 is inputted to the control unit 4. Further, a vehicle speed sensor 13 is provided for detecting the speed of the motor vehicle, wherein the output signal of this sensor 13 is also inputted to the control unit 4.
The driving mechanism 61 of the throttle actuator 6 is usually constituted by a DC motor, a stepping motor or the like. The electromagnetic clutch 62 is of a friction disk type. The relation between the depression of the accelerator pedal 14 and the opening degree of the throttle valve 12 is controlled by the actuator control circuit 41 in accordance with a predetermined function so as to ensure a comfortable driving of the motor vehicle.
Further, the throttle valve 12 is controlled by the throttle actuator 6 in response to the output signal of the actuator control circuit 41 so that the detected vehicle speed coincides with a desired or target vehicle speed commanded by the driver.
By controlling the opening degree of the throttle valve 12 in accordance with a predetermined function or through a vehicle-speed feedback loop by using the throttle actuator 6 without coupling directly the throttle valve to the accelerator pedal, there can be assured a high freedom in the running control of the motor vehicle as well as a high drivability thereof.
With the arrangement of the conventional control apparatus described above, an abnormal running state of the motor vehicle which may take place upon the occurrence of failure or fault in the accelerator pedal depression sensor 1 or the throttle actuator 6 is prevented by providing the fault detecting circuit 42 which responds to the detection of a the fault by interrupting the electric power supply to the electromagnetic clutch 62 through the clutch control circuit 43 and deenergising the clutch 62 for thereby releasing the throttle valve 12 from the driving mechanism 61 of the throttle actuator 6 to allow the throttle valve 12 to be fully closed under the influence of the return spring 11.
However, in practical applications, difficulty is encountered to perform the fault detection in a perfectly satisfactory manner. In particular, when a fault occurs simultaneously in both the accelerator pedal depression sensor 1 and the clutch control circuit 43, the engine speed may increase uncontrollably, because the throttle valve 12 can no more be fully closed in that case and because the braking as applied is not effective when the throttle valve 12 is opened.